A number of hosts for gene expression and methods of transformation have been disclosed in the prior art. Bacteria are often mentioned e.g. Escherichia coli. E. coli is however a micro-organism incapable of secretion of a number of proteins or polypeptides and as such is undesirable as host cell for production of protein or polypeptide at the industrial level. An additional disadvantage for E. coli, which is valid also for bacteria in general, is that prokaryotes cannot provide additional modifications required for numerous eukaryotic proteins or polypeptides to be produced in an active form. Glycosylation of proteins and proper folding of proteins are examples of processing required to ensure an active protein or polypeptide is produced. To ensure such processing one can sometimes use mammalian cells; however, the disadvantage of such cells is that they are often difficult to maintain and require expensive media. Such transformation systems are therefore not practical for production of proteins or polypeptides at the industrial level. They may be cost efficient for highly priced pharmaceutical compounds requiring relatively low amounts, but certainly not for industrial enzymes.
A number of fungal expression systems have been developed e.g. Aspergillus niger, Aspergillus awamori, Aspergillus nidulans, Trichoderma reesei. A number of others have been suggested but for various reasons have not found wide-spread acceptance or use. In general terms the ideal host must fulfil a large number of criteria:                The ideal host must be readily fermented using inexpensive medium.        The ideal host should use the medium efficiently.        The ideal host must produce the polypeptide or protein in high yield, i.e. must exhibit high protein to biomass ratio.        The ideal host should be capable of efficient secretion of the protein or polypeptide.        The ideal host must enable ease of isolation and purification of the desired protein or poly-peptide.        The ideal host must process the desired protein or polypeptide such that it is produced in an active form not requiring additional activation or modification steps.        The ideal host should be readily transformed.        The ideal host should allow a wide range of expression regulatory elements to be used thus ensuring ease of application and versatility.        The ideal host should allow use of easily selectable markers that are cheap to use.        The ideal host should produce stable transformants.        The ideal host should allow cultivation under conditions not detrimental to the protein or polypeptide product, e.g. low viscosity, low shear.        
WO 96/02563 and U.S. Pat. Nos. 5,602,004, 5,604,129 and 5,695,985 to Novo Nordisk describe the drawbacks of Aspergillus and Trichoderma systems and suggest that cultivation conditions for other fungi may be more suited to large scale protein production. The only examples provided for any transformed cultures are those of Myceliophthora thermophila, Acremonium alabamense, Thielavia terrestris and Sporotrichum cellulophilum strains. The Sporotrichum strain is reported to lyse and produce green pigment under fermentation conditions not leading to such results for the other strains. A non-sporulating mutant of Thielavia terrestris is described as being the organism of choice by virtue of its morphology. However it is also stated that the protoplasting efficiency of Thielavia and Acremonium (whereby the Acremonium strain used was the imperfect state of the Thielavia strain used) is low and that hygromycin was not useful as a selection marker. A large number of others are suggested as being potentially useful by virtue of their morphology but no transformation thereof is described. The suggested strains are Corynascus, Thermoascus, Chaetomium, Ctenomyces, Scytalidium and Talaromyces. The transformed hosts are mentioned as only producing low levels of the introduced Humicola xylanase with Thielavia producing the lowest amount; however, the information is ambiguous and could actually infer Thielavia was the best embodiment. The nomenclature of this reference is based on the ATCC names of Industrial Fungi of 1994. Thus it is apparent that no high degree of heterologous expression was achieved and in fact no positive correlation could be derived between the postulated morphology and the degree of expression. If any correlation could be made, it was more likely to be negative. According to the 1996 ATCC fungal classification Sporotrichum thermophilum ATCC 20493 is a Myceliophthora thermophila strain. Currently the strain is still identified as Myceliophthora thermophila. The unpredictability of the art is apparent from these recent disclosures.
WO 97/26330 of Novo Nordisk suggests a method of obtaining mutants of filamentous fungal parent cells having an improved property for production of heterologous polypeptide. The method comprises first finding a specific altered morphology followed by assessing whether a transformant produces more heterologous polypeptide than the parent. The method is illustrated only for strains of Fusarium A3/5 and Aspergillus oryzae. The method is suggested to be applicable for Aspergillus, Trichoderma, Thielavia, Fusarium, Neurospora, Acremonium, Tolyplocadium, Humicola, Scytalidium, Myceliophthora or Mucor. As stated above, the unpredictability in the art and also the unpredictability of the method of the cited application do not provide a generally applicable teaching with a reasonable expectation of success.
In WO 00/20555, we have described an alternative fungal expression system with the simplicity of use of the above-mentioned Aspergilli and Trichoderma fulfilling the above requirements. The new system provides the additional advantages that transformation rates are higher than those for the frequently used Trichoderma reesei system. In addition the culture conditions offer the additional bonus of being advantageous for the polypeptide product.